In consideration of the environment, a high power battery with high energy density is demanded as a power source for a hybrid car that can use energy efficiently. Since lithium secondary batteries with the use of nonaqueous electrolytic solution are high in battery voltage and high in energy density, these are promising as a battery for use in automobile. When a lithium battery is used as a secondary battery for automobile, excellent input-output characteristics, operational stability in a wide range of temperature, and long life characteristics are demanded. For improvement of the input-output characteristics, optimization of electrode structure by enhancing electron conductivity in an electrode is being studied. On the other hand, for widening of the range of operational temperature, the development of functional electrolytic solution in which a decrease in lithium ion transport characteristics at low temperatures is suppressed is in progress. Further, for longer lasting life, the development of lamellar positive electrode active material that keeps a stable crystal structure even after undergoing charge and discharge cycles and a long period of storage is in progress.
The structure of a positive electrode sheet formed of a conductive material and a positive electrode active material is closely related to input-output characteristics of the battery. Generally, the conductive material is uniformly dispersed in the positive electrode sheet to form a conductive network, and the electrode resistance is lowered, thus obtaining a high power battery. Until now, it has been attempted to add various powder graphite in a positive electrode as a conductive carbon material.
In JP-A No. 14582/1995, it is shown that electrode resistance is lowered by using carbon nanotubes having high electron conductivity as a conductive material for a positive electrode.
The function that is necessary for the conductive material used in a positive electrode includes a liquid retention property of keeping an electrolytic solution in the vicinity of a positive electrode active material. When discharge is performed in a short time, lithium ion has to be instantly supplied from the electrolytic solution present in the vicinity of the positive electrode active material, and therefore, the conductive material retains the electrolytic solution in its micropores. In conductive materials such as carbon black, the electrolytic solution or lithium ion can easily move in and out of the end face of lamellar carbon.
On the other hand, in order to decrease the internal resistance of the electrode to a significant degree, it is necessary to form a conductive network among particles of the positive electrode active material, and long carbon nanotubes that can interconnect the particles of the positive electrode active material are required. Generally, the diameters of primary particles of the positive electrode active material used in a lithium battery are from submicron to several microns, and there are spaces of about several micrometers among the primary particles. Thus, carbon nanotubes having a length of about several micrometers are necessary for formation of the conductive network among the particles of the positive electrode active material. In other words, when carbon nanotubes are used in the positive electrode for lithium battery, there are lengths of carbon nanotubes that are suitable for moving-in and moving-out of the electrolytic solution or lithium ion and for the formation of the conductive network in the positive electrode active material.
However, carbon nanotubes are made of a carbon material having a cylindrical structure, and therefore, the electrolytic solution or lithium ion can move in and out only from both ends of the nanotubes. Accordingly, it is necessary that a shearing force is applied to the carbon nanotubes to cleave the tubes partially, thereby increasing opening portions. However, carbon nanotubes having a diameter smaller than 10 nm are cut in this process, giving rise to a shorter average length. It has not been disclosed in JP-A No. 14582/1995 that the formation of conductive network in the positive electrode active material and the liquid retention property of keeping the electrolytic solution were both fulfilled at the same time.
On the other hand, it is disclosed in JP-A No. 323142/2000 that the electrode resistance of a positive electrode is reduced by the use of a fibrous carbon material such as vapor growth carbon fiber (VGCF) as the conductive material. Since fibrous carbon material such as VGCF has a diameter larger than 100 nm, it is difficult to shear the fibrous carbon material. Accordingly, it is hard to mix and disperse the fibrous carbon material that has once aggregated and become clumpy in a positive electrode active material. When the aggregated fibrous carbon material and the positive electrode active material are mixed with each other and then slurry is prepared by adding an organic binder and an organic solvent to make an electrode coated with this slurry, the aggregated fibrous carbon material having high liquid absorbability absorbs the organic solvent and the binder locally in the electrode, thereby forming an ununiform electrode.
Further, it is disclosed in JP-A No. 86174/2003 that a complex particles attached with a carbon material on the surface of a positive electrode active material are prepared by mixing the positive electrode active material with 0.5 to 6% by weight of the carbon material having a BET specific surface area equal to or larger than 29 m2/g and that the use of these particles can fulfill both of the reduction in the resistance of an electrode and the retention of discharge capacity. When the mixing ratio of the carbon material is 6%. by weight or higher, the discharge capacity is decreased.